1. Field of the Invention
The present invention relates to the field of tunnels or chambers supplied with gas in order to carry out operations under an atmosphere, such as, for example, heat-treatment operations, soldering operations, operations of packaging food products or cooling operations using cold gases.
The present invention relates to:
a method of regulating the content of a given component of the atmosphere in a chamber; PA1 a process for supplying gas to a chamber, implementing this regulating method; and PA1 the application of this method and of this process to the cases of heat-treatment ovens or soldering ovens, or else of ovens or machines for soldering/tinning electronic components, or alternatively of chambers for the packaging or cooling of food products. PA1 the flow rate variations are, on the one hand, the source of turbulence, promoting the creation of air intakes into the chamber (modification of the equilibrium in the gas movements) and therefore degradation of the quality of the atmosphere in this chamber; PA1 as described previously, a variation in the convective heat transfer carried out inside the chamber, and therefore in the quality of the soldered products obtained on leaving this chamber. PA1 a) the content of the given component of the atmosphere in the chamber is measured at at least one point in the chamber; PA1 b) a comparison is made with at least one predetermined control value for the content of the said component of the atmosphere in the chamber at the said point; and PA1 c) where necessary, the pressure of the gas at one of the points in the network is varied, depending on the result of this comparison, preferably one of the following points: PA1 c1) a solenoid valve with proportional control is placed at the relevant point in the network; PA1 c2) the pressure of the gas near this point is measured downstream of the solenoid valve; PA1 c3) the measurement of the content of the given component, obtained during step a), is transmitted to a data acquisition and processing unit capable, depending on the said content measurement and on the comparison made during step b), of generating a pressure control value Cp; and PA1 c4) this pressure control value Cp is transmitted to a means capable of comparing the pressure measurement of step c2) with the control value Cp and of accordingly modifying the amount by which the solenoid valve is open in order, where necessary, to bring the said pressure back to the level of the control value Cp. PA1 c1) that line of the network where the relevant point lies, whether it be a primary line or a feed pipe, is subdivided at the relevant point in the network where the pressure is made to vary, into at least three bypass lines, each of these bypass lines being equipped with a solenoid valve and with a pressure-reducing valve lying downstream of the solenoid valve, the downstream end of each of the bypass lines being connected again to the relevant line of the network; PA1 c2) each of the three pressure-reducing valves is preset to a predetermined pressure level; and PA1 c3) the measurement of the content of the given component, obtained during step a), is transmitted to a data acquisition and processing unit capable, depending on the measured content and on the comparison made during step b), of selectively opening one of the solenoid valves in order to allow passage of the gas only into the bypass line whose solenoid valve has been opened. PA1 in a first bypass line, a pressure-reducing valve preset to a first pressure level P.sub.s corresponding to a gas supply flow rate, downstream of the pressure-reducing valve, which may be called the "standby" flow rate Q.sub.s ; PA1 in a second bypass line, a pressure-reducing valve preset to a pressure level P.sub.prod corresponding to a gas flow rate, downstream of the pressure-reducing valve, which may be called the "production" flow rate Q.sub.prod ; and PA1 in the third bypass line, a pressure-reducing valve preset to a pressure P.sub.fill corresponding to a gas supply flow rate, downstream of the pressure-reducing valve, which may be called the "filling" flow rate (or the "turbo" flow rate) Q.sub.fill. PA1 when starting up the chamber (whether this be, for example, a heat-treatment oven or an oven or machine for the soldering/tinning of electronic components), a first, filling phase is carried out during which the chamber is supplied with gas, the "filling" bypass line being in operation (the other bypass lines therefore being closed). PA1 after the predefined filling time t.sub.fill has elapsed, the system is then ready to allow entry of the products to be treated and to start measuring the content of the relevant component of the atmosphere in the chamber. PA1 as soon as a component or product to be treated is detected (for example by a detection cell) at the entrance of the chamber (i.e. at a greater or lesser distance upstream on the conveyor for taking the components to the entrance of the chamber), the system is then in a phase which may be called the "production" phase in which the data acquisition and processing unit switches the system, this time, to the "production" bypass line, opening the solenoid valve corresponding to the third bypass line. PA1 after the production flow-rate conditions have been established, a measurement of the content of the relevant component of the atmosphere in the chamber may advantageously be taken, in order to allow entry of the products (which, it will be recalled, are detected at a greater or lesser distance upstream of the chamber), only if the measured content is below a threshold S.sub.prod ; PA1 advantageously, if at any moment during the "production" phase the measurement of the content of the relevant component of the atmosphere in the chamber reaching the data acquisition and processing unit is above a first predefined threshold S1 (S1 is preferably greater than or equal to S.sub.prod), the unit will switch the system to the "filling" bypass line and will wait until the measured content comes back below a second predetermined threshold (S2), equal to or less than the threshold S1, in respect of the content of the component of the atmosphere in the chamber, allowing the system to be returned to the "production" mode.
By way of illustration, such "tunnels" or "chambers" might thus, for example, consist of heat-treatment ovens, soldering ovens, ovens for the reflow soldering of electronic components on circuits, or else machines for the wave soldering/tinning of electronic components, whether these machines are designed as tunnels which are completely sealed over their entire length or they are provided with shrouding systems extending above the solder bath or to a greater or lesser extent largely around this solder bath in order to enclose, for example, the preheat zone.
2. Description of Related Art
Two problem areas encountered by users of such plants are:
1) The Instability of the Atmosphere Employed:
Mention may be made here of the widespread example of applications in which the user tries to stabilize the residual oxygen content of the nitrogen-based atmospheres employed in these chambers.
This problem area of stabilizing the atmosphere employed is, of course, closely linked with the user's desire for excellent reproducibility of the quality of the components or products treated in the chamber in question.
Still by way of illustration, variations in the quality of the components treated in the chamber may, for example, be related, in the case of inert-type atmospheres, to the oxidizability of the atmosphere (therefore to the level of residual oxygen or other oxidizing gas in this atmosphere), or else to instabilities in the heat transfer employed in the chamber due to the observed instabilities in the atmosphere employed, whether this be instability in terms of composition or in terms of gas flow rates.
The observed instabilities in the atmosphere are more generally related to the production rate of the chamber or to the external conditions surrounding the oven, for example possible droughts.
In this first problem area, mention may be made of the case of ovens for the reflow soldering of electronic components on circuits, so-called "convective" ovens, which therefore carry out the heat transfer necessary for soldering the component in an essentially convective mode, circulating very large volumes of gas in each zone of the oven.
It is known in fact that at least some of the zones of such convective ovens (in particular the hot zones) operate with a gas recirculating (recycling) system. In these recirculating zones, only a "relatively low" volume is regularly added, in order to compensate for the gas losses involved, in particular via the extraction chimneys or the inlets/outlets of the oven.
These convection reflow ovens are therefore characterized, on the one hand, by very large consumptions of gas but also, on the other hand, by great difficulties encountered by users in regulating these ovens, any modification of the gas flow in certain zones of the oven having considerable repercussions on the distribution equilibrium of the gas movements in the oven (turbulence) and on the existing heat profile (and therefore necessarily on the quality of the components produced).
Studies carried out by the Applicant on this subject show, in fact, that these ovens must be adjusted in the production phase in order to perform correctly and be representative of the subsequent operating conditions. It is then necessary to modify the flow rate selectively supplying such or such a zone of the oven manually, thereby giving rise to considerable variations in the flow rate in the other zones of the oven which require manual readjustments to be made zone by zone.
In order to illustrate these considerations, let us thus take the example of the adjustment of such a convective oven, supplied overall with a flow rate of 50 Nm.sup.3 /h of nitrogen, this adjustment being carried out, for example, in a very precise and stable manner using a mass flow rate regulator present on the upstream side of the plant (upstream of the set of lines serving all the zones of the oven). Should it be necessary, in the adjustment phase, to drop the flow rate in one of the zones, for example from 20 Nm.sup.3 /h to 5 Nm.sup.3 /h, the regulator will continue to supply the oven with an overall flow rate of 50 Nm.sup.3 /h, and therefore distributing the 15 Nm.sup.3 /h excess over all the other lines. It is therefore found that the flow rate in the other zones will be irremediably modified, thus also modifying the heat transfer which occurs therein, and therefore the overall quality of the components produced (it will be recalled that the oven is necessarily adjusted during production, i.e. "live").
2) Savings in Gas:
Some of the plants mentioned therefore consume a very large amount of gas, and it is therefore a constant preoccupation of users to try to decrease the flow rates employed in the chamber, i.e. to find the best compromise between the gas flow rate used and the quality of the components produced.
Among the approaches generally adopted for making such savings in gas consumption are decreasing the gas flow rates used depending on the production rate of the components (for example, to manage the production breaks), or optimizing the composition of the atmosphere used, for example the residual oxygen content tolerated in the atmosphere in the chamber depending on the characteristics of the components treated in this chamber (for example, the level of oxidizability of the metals treated).
Considering the example of nitrogen-based atmospheres, having a controlled oxygen content, one of the existing approaches for solving these problems may be termed the "flow-rate-regulation atmosphere control" approach.
According to this approach, a certain adjustment (flow rate and distribution of the gas injections into the chamber) and a chosen supply of nitrogen, of a given oxygen purity, are employed, these being sufficiently limited for there effectively to exist variations or perturbations in the operation of the chamber which can cause the maximum permissible residual oxygen content of the atmosphere in this chamber to be exceeded.
The gas supply chosen is then used in combination with a regulating system making it possible, when the measured residual oxygen content of the atmosphere in the chamber exceeds a given threshold, because of the abovementioned variations or perturbations, to increase the overall flow rate of gas in the chamber so as to decrease this flow rate only when the measured oxygen content of the atmosphere in the chamber has gone back below the threshold.
To be sure, this approach makes it possible to adapt the purity of the gas supply used and the flow rate employed to the actual requirements of the practical operation in the chamber, but may lead, in the long run, to a relatively high overall gas consumption. As mentioned above, the applicant has furthermore observed, in practice, the fact that some plants, such as convective-reflow soldering ovens, are very difficult to adjust in terms of flow rate, and the variations in the flow rate employed in certain zones of the oven, such as conventionally in the key zone of the reflow peak in such soldering operations, lead to considerable variations in the other zones of the oven, and therefore overall to instability phenomena in the process itself: